1. Field of the Invention
The present invention generally relates to data transfer techniques for data processing systems and more particularly relates to data transfer over extended distances.
2. Description of the Prior Art
It has been known for some time to transfer digital data between peripheral equipments and host computers. The earliest and most popular medium for such transfers is electrical energy, which flows within a circuit including portions of the host computer, peripheral devices, and interconnecting electrical cables. These data transfers may be serial in nature, which transmit one data bit at a time, or parallel, in which a number of data bits are transmitted simultaneously.
For a given switching speed of an individual electrical circuit, the parallel approach is inherently faster because it transfers multiple data bits simultaneously. This has meant that serial transmissions have tended to be limited to low data rate information paths or to long distances wherein the cost for parallel transmission is prohibitively expensive. Therefore, most modern day data transfers between host computers and associated peripheral devices utilize parallel electrical transmission. A typical protocol for such transmissions is the popular Block Multiplexer Channel (i.e. BMC) utilized by Unisys Corporation. This highly efficient approach transfers data as parallel bytes (along with parity bits) over a first parallel cable and control signals over a second parallel cable. Because these two cables transfer data and control signals only in one direction, a second pair of cables is usually needed for transfers in the opposite direction. The technique provides an effective transfer rate in each direction of nearly 4.5 MB/sec.
A second medium which is gaining popularity for data transmission is fiber optics. In this approach, the digital data is converted to pulses of light which are transferred over a special light conducting fiber optic cable. Because this transmission medium does not experience the same distributed capacitance which delays electrical transmissions, higher data rates can generally be achieved for a given transmission energy. In fact the data transmission rates tend to be sufficiently high, that serial fiber optic transmissions can be utilized to replace parallel electrical transmissions for many host computer to peripheral device transfers.
Regardless of the transmission medium, particular difficulty is experienced with transmission over extended distances. This occurs because of the finite additional time per unit of distance required to transfer the signal. For most practical standardized interfaces, such as Unisys BMC referenced above, a maximum practical distance is specified (e.g. 500 feet). Such a standard maximum length permits defining efficient software and hardware applicable to the majority of interfaces, which transfer data over short distance (i.e. less than 500 feet). A different or supplementary protocol is defined for extended distances (i.e. in excess of 500 feet). This second protocol accommodates the delays associated with the longer transfer time.
A second characteristic of long distance data transfer is signal loss as a function of distance. Whereas this factor also occurs with optical transmission, it is clearly most pronounced with the electrical medium. One component of this electrical loss is purely resistive in nature. Additional driver voltage may be used to compensate. However, a second reactive component (usually capacitive) tends to complicate the problem. The result is that extended distance electrical data interfaces usually employ different and much higher power circuitry in an attempt to lessen the time delays and adverse signal to noise ratios. These non-standard protocols coupled with the non-standard hardware and software present system integration, power dissipation, and reliability problems.
At the corresponding data rates using the optical medium and current fiber optic techniques, extended distance transmission produces time delays over and above shorter distance transmission. However, the corresponding signal losses are not nearly as severe as with the electrical medium. Therefore, systems employing optical transmission techniques up to several miles need compensate only for the additional time delays but need not be concerning with signal losses. However, prior art systems provide this compensation only within the framework of protocols designed expressly for optical transmission. This produces incompatibility with existing hardware, software, and interface standards.